Living organisms are composed of killions of organic compounds each having a unique structure what element

Carbon

Explanation:

Carbon has the ability to form a wide range of compounds. It has four valence electrons and thus can form single, double and triple bonds. It also has a tendency to bond with itself, forming long chains or cyclic structures. These bonding capabilities allow for many different combinations, resulting in the possibility of multiple unique compounds.

For example, a 4 carbon compound with hydrogens attached peripherally can still have 3 alternatives; it can be an alkane, an alkene or an alkyne, due to the fact that carbon can form different types of bonds.

c. is the correct answer